WHAT: Defining a population is multi-faceted
- A group of organisms from one species…
- abundance
- sex ratios
- age structure
- density
- Defining the population boundary…
- a lake
- ecosystem
- country or state
- Aphid population: leaf, plant, grassland, ecosystem?
- depends on the study and the questions

WHY: Understanding population structure (Demography)
- is population size ↑ ←→ ↓ ?
- Age of a population, peak reproductive ages
- Frequency of reproduction, reproductive effort
- Sex ratios determining mating potential
- Measuring, predicting and evaluating population sizes guides decisions
- conservation, pest management, harvesting

Case Study: Deer population size


Case Study: Deer population density matters too…


Case Study: Zombie deer and population density


WHO: Definition of an individual can be tricky
- Unitary organism: forming a single entity
- Modular organisms: colonies, etc.
- bacteria, algae, corals
- plants: ferns grow by rhizomes
- Some modular organisms may themselves be populations

HOW: Estimates from representative samples
- Lessons from Lab:
- Census techniques
- Mark & Recapture
- Spatial component
- Temporal component
- Most methods are indirect estimates

Populations dynamics: Movement (dispersal and migration)

Life History: More than just birth and death

Reproductive effort and life cycles differ greatly

Semelparous: One-off reproduction


Species with short life cycles: Annuals & Ephemerals


Life History: Sexual conflict
- When and how to spend your resources
- reproduction comes at a cost of growth
- flowers + seeds = ↓ leaves and roots
- are enough resources available to support pregnancy in animals?
- surviving vs growing the population…
- Life histories of males and females may differ

Populations and life cycles: Dormancy
- Annual organism may spend part of the year dormant
- Seeds, Spores, Cysts, etc.
- Thousands of seeds per square meter of soil
- Dormancy can last for long periods of time
- protection from harsh environments
- How does this affect population estimates?
- Reproductive effort (seeds) and offspring may not be synced


- Populations are composites of individual Life Histories
- who survived from previous year
- new individuals (births/immigration)
- survivability of offspring
Survirvorship =
Populations dynamics: Monitoring birth and death
- Measuring rates of birth and death can tell us if a population is growing or shrinking
- Cohort Life tables: follow the fate of individuals from a single cohort
- survivorship
- difficult for mobile animals
- Static life table: count # of survivors at different ages
- Tells us a lot about age structure of a population

r/K selection theory: Growth and reproduction strategies
- r-selected species: “cheap” offspring
- K-selected species: “expensive” offspring
- These are the 2 ends of a spectrum



Populations with unlimited resources: Exponential growth


Population growth regulation
- Density-dependent
- competition
- predation
- disease
- resources
- Density-independent
- weather events
- disturbances
- Carrying Capacity:
- environmental resistance leads to Logistic growth
